Denkir II was a painting, depicting the titular planet from the perspective of its moon. The painting was displayed aboard the USS Enterprise-D. The painting was seen in officer quarters and in guest quarters. (TNG: "The Naked Now", "Too Short a Season")
The painting, and another, were created by Rick Sternbach while writing a non-Trek related SF story, and later provided by him when set artwork was needed.  It is unclear if a real Denkir II existed, though if it did it might be related to the Denkir depicted on some maps or to any of several other things also called "Denkir". The painting might even depict something entirely differently, or might be named differently, in-universe.
A copy of this painting was first seen in Beverly Crusher's quarters in "The Naked Now". It was also seen Jean-Luc Picard's quarters in "Suddenly Human", "Legacy", "Devil's Due", and "Qpid". In William T. Riker's quarters in "Lonely Among Us", Worf's quarters in "Peak Performance" and "Redemption", and in B.G. Robinson's quarters in "The Outrageous Okona".